Line, one nailed by a fool one's played (9)
I believe the answer is:
accordion
'one's played' is the definition.
'line one nailed by a fool' is the wordplay.
'line' becomes 'cord' (cord is a kind of line).
'one' becomes 'i' (Roman numeral).
'nailed by' indicates putting letters inside.
'fool' becomes 'con'.
'cord'+'i'='cordi'
'a'+'con'='acon'
'cordi' inserted within 'acon' is 'ACCORDION'.
(Other definitions for accordion that I've seen before include "Squeeze box" , "squeeze it while operating keys" , "Cad I croon about a musical instrument" , "Squeeze-box" , "It's played" .)